This invention relates to resonant audio circuits and more specifically relates to parametric equalizers used in audio circuits.
Parametric circuits capable of adjusting one or more parameters have found increasing acceptance in audio devices. Such circuits can be used to equalize room accoustics, compensate for a deficiency in an overall audio system, change the quality of sound of a specialized or non-linear amplifier, such as a guitar amplifier, or give a different character or timbre to a signal generated by an electronic musical instrument, such as a synthesizer or electronic organ.
Parametric circuits generally try to achieve these results by changing one or more parameters in order to control an input audio electrical signal. Such circuits generally have a response characterized by the parameters of height, center frequency and width. Parametric equalizers are capable of adjusting all three of these parameters, whereas graphic equalizers generally adjust only the height parameter. These parameters are more completely defined in a paper by one of the coinventors, Robert A. Moog, entitled "Graphic and Parametric Equalizers" published in November, 1977 issue of Contemporary Keyboard Magazine, and incorporated by reference.
By controlling the foregoing parameters, audio electrical signals can be altered in order to provide a variety of musical effects. For example, particular ranges of frequencies can be emphasized or de-emphasized in order to provide additional color to the timbre of the resulting sounds.
Although parametric circuits have been devised in the past, each has exhibited deficiencies making it unsuitable for use in audio circuits, especially audio circuits intended for electronic musical instruments. For example, such circuits have been excessively noisy, even when the height parameter is neutral (neither boosted nor cut). In addition, some of the more inexpensive circuits have a residual gain when an input audio signal is widely displaced from the resonant frequency of the circuit. As a result of this characteristic, both the parametric controls and the overall volume or amplitude control of the circuit must be simultaneously adjusted in order to control the amplitude of the resulting electrical signal. The use of such a circuit in an electronic musical instrument compromises the performer's ability to control the resulting sound. In addition, prior circuits have been arranged so that manipulation of the width control also affects the height parameter. This characteristic is also undesirable for applications in electronic musical instruments.
Circuits overcoming some of the foregoing deficiencies are known in the art in the form of graphic equalizers. Although these circuits overcome some of the foregoing deficiencies, they are excessively expensive to manufacture. One such circuit utilizes a difference amplifier having a series of potentiometers connected in parallel across its inputs. The wipers of the potentiometers are connected to antiresonant circuits having particular frequencies, generally spaced an octave apart. In order to cover the entire audio sound spectrum, a large number of such circuits must be utilized.
The applicants have overcome the deficiencies of the prior art by devising an improved frequency sensitive circuit for use in connection with the difference amplifiers previously used in graphic equalizer circuits. By employing this improved circuit, the resonant frequency can be adjusted over a substantial portion of the audio range and the height and width parameters can also be controlled with a minimum of circuitry and expense. In addition, all of the above noted deficiencies of the prior art circuits can be overcome.